Display devices of the TN type and STN type in which an electro-optical effect is applied to a nematic liquid crystal material have been put into practical use. Display devices comprised of a liquid crystal material having strong dielectric characteristics have also been proposed. These devices require a polarized light filter and an orientation treatment. On the other hand, the dynamic scattering effect and phase transition effect can be employed to provide a liquid crystal device in which a polarized light filter is not required and the liquid crystal material is dispersed.
Recently, it has been proposed that the refractive index of liquid crystal materials be electrically controlled to provide transparent conditions and white-cloudy conditions, so that a polarized light filter and an orientation treatment are not required. In this method, when voltage is applied, the orientation of liquid crystals corresponds to the direction of the electrical field and the refractive index of liquid crystals with an ordinary ray is made to correspond to the refractive index of a polymer medium, so that transparent conditions are formed. The light scattering conditions caused from the irregular orientation of liquid crystal molecules are formed when no voltage is applied.
This method is proposed, for example, in Japanese Patent Laid-Open Sho 58-501,631 in which liquid crystals are incorporated in polymer capsules, in Japanese Patent Laid-Open Sho 61-502,128 in which liquid crystal material and light-curable or heat-curable material are mixed and cured under phase separation to form liquid crystal droplets, and in Japanese Patent Laid-Open Sho 59-226,322 in which a solvent is removed from a mixture of polymer and liquid crystal, both of which are dissolved in the solvent, to form phase-separated conditions of polymer/liquid crystals.
The liquid crystal materials used in the invention which are related to those of the references are ones having a --CN group at the terminal end of the molecular structure, such as cyano biphenyl compounds and cyano pyrimidines. Such liquid crystal materials are disclosed in Japanese Patent Laid-Open Hei 2-272,422 to 2-272,424, 2-75,688 and 2-85,822.
However, the liquid crystal compounds having a --CN group at the terminal end which are conventionally used for preparing the liquid crystal display devices of the polymer dispersion type have high reactivity and tend to attract impurities in the system because of strong polarization of the --CN group. Therefore, a liquid crystal display device of the polymer dispersion type having high charge holding characteristics (90% or more) cannot be provided through conventional production processes because the liquid crystal materials are often brought into contact with the other materials. In the present application, the term "charge holding characteristics" means voltage holding ratio as is defined, for example, by S. Matsumoto et al., Liq. Cryst. 5, 1320 (1989).
In particular, in methods wherein curable compounds are cured from a mixture thereof with liquid crystal compounds so as to separate liquid crystal materials from the cured materials, the liquid crystal materials are sometimes partly damaged during the curing process. This occurs because both the liquid crystal compounds and polymerizable active compounds exist together in one cell and the liquid crystal compounds are highly reactive. Therefore, resultant liquid crystal devices sometimes exhibit poor charge holding characteristics, which is especially true in cases where the liquid crystal mixture is based on compounds with a terminal cyano group, Schiff's base LC compounds, and the like.
On the other hand, it may be thought that in order to get rid of hysteresis, a photocurable resin of low surface interaction can be added. However, because such a resin has a different SP value (solubility parameter, see below) from that of conventional liquid crystals, the resins and the liquid crystals are not soluble in each other, so that they can not be put to practical use.
In methods wherein liquid crystals and photo curable resins are mixed uniformly and then phase-separated by photopolymerization, there includes a step of mixing the liquid crystal materials with the photocurable resins. Therefore, when the compatibility between the liquid crystal materials and the resins is poor, it is necessary to raise the mixing temperature in order to form a uniform LC and resin mixture. When photopolymerization is carried out at such a high temperature to separate phases, the polymerization speed becomes high, so that the phase separation is started in nearly isotropic phase conditions. Therefore, the dispersed particles of liquid crystal material become small and as a result, high driving voltage is required.
The present invention has been achieved in consideration of the above circumstances.
An object of the present invention is to provide a liquid crystal composite layer of the polymer dispersion type which can meet all the most important requirements for the liquid crystal display devices of the polymer dispersion type, i.e., low driving voltage, high contrast and high charge holding characteristics.
Another object of the present invention is to provide a liquid crystal optical device comprised of a liquid crystal composite layer of the polymer dispersion type.
Another object of the present invention is to provide a liquid crystal material which does not exhibit the shortcomings of conventional materials based on compounds with a terminal cyano group and does not undergo degradation or degradation only to an acceptable degree when preparing the liquid crystal composite layer of the present invention via photopolymerization.
Another object of the present invention is to provide a method for preparing said liquid crystal composite layer.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.